Cryogenic air separation is a very energy intensive process because of the need to generate high pressure, very low temperature air streams and the large amount of refrigeration needed to drive the process. In a typical cryogenic air separation plant, an incoming feed air stream is passed through a main air compressor (MAC) arrangement to attain a desired intermediate discharge pressure and flow. Prior to such compression, dust and other contaminants are typically removed from the incoming feed air stream via an air filter typically disposed in an air suction filter house. The filtered air stream is compressed in a multi-stage MAC compression arrangement typically to a minimum pressure of about 6 bar and often at higher pressures. The compressed, incoming feed air stream is then purified in a pre-purification unit to remove high boiling contaminants from the incoming feed air stream. Such a pre-purification unit typically has beds of adsorbents to adsorb such contaminants as water vapor, carbon dioxide, and hydrocarbons. In many air separation plants the compressed, purified feed air stream or portions thereof are further compressed in a series of booster air compressor (BAC) arrangements to even higher discharge pressures. In conventional air separation plants, the MAC compression arrangements are located upstream of pre-purification unit whereas the BAC arrangements are located downstream of pre-purification unit.
The compressed or further compressed, purified feed air streams are then cooled and separated into oxygen-rich, nitrogen-rich, and argon-rich fractions in a plurality of distillation columns that may include a higher pressure column, a lower pressure column, and optionally, argon column (not shown). As indicated above, prior to such distillation the compressed, pre-purified feed air stream is often split into a plurality of compressed, pre-purified feed air streams, some or all of which are then passed to a multi-stage BAC compression arrangement to attain the desired pressures required to boil the oxygen produced by the distillation column system. The plurality of compressed, pre-purified feed air streams including any further compressed, pre-purified feed air streams are then cooled within the primary or main heat exchanger to temperatures suitable for rectification in the distillation column system. The sources of the cooling the plurality of feed air streams in the primary heat exchanger typically include one or more waste streams generated by the distillation column system as well as any supplemental refrigeration generated by the cold turbine and warm turbine arrangements, described below.
The plurality of cooled, compressed air streams are then directed to two-column or three column cryogenic air distillation column system which includes a higher pressure column thermally linked or coupled to a lower pressure column, and an optional argon column. Prior to entering the higher pressure column and lower pressure columns, any liquid air streams may be expanded in a Joule-Thompson valve to produce still further refrigeration required for producing the cryogenic products, including liquid oxygen, liquid nitrogen and/or liquid argon.
In air separation units designed to produce a large amount of liquid products, such as liquid oxygen, liquid nitrogen and liquid argon, a large amount of supplemental refrigeration must be provided, typically through the use of Joule-Thompson valves, described above, cold turbine arrangements and/or warm recycle turbine arrangements. Cold turbine arrangements are often referred to as either a lower column turbine (LCT) arrangement or an upper column turbine (UCT) arrangement which are used to provide supplemental refrigeration to a two-column or three column cryogenic air distillation column system. On the other hand, a warm recycle turbine (WRT) arrangement expands a refrigerant stream in a warm turbo-expander with the resulting exhaust stream, cooled via expansion of the refrigerant stream, imparting supplemental refrigeration to the cryogenic air distillation column system via indirect heat exchange with the pre-purified, compressed feed air in the primary heat exchanger or in an auxiliary heat exchanger.
In the LCT arrangement, a portion of the pre-purified, compressed feed air is further compressed in a BAC compression arrangement, partially cooled in the primary heat exchanger, and then all or a portion of this further compressed, partially cooled stream is diverted to a turbo-expander, which may be operatively coupled to and drive a compressor. The expanded gas stream or exhaust stream is then directed to the higher pressure column of a two-column or three column cryogenic air distillation column system. The supplemental refrigeration created by the expansion of the diverted stream is thus imparted directly to the higher pressure column thereby alleviating some of the cooling duty of the primary heat exchanger.
Similarly, in the UCT arrangement, a portion of the purified and compressed feed air is partially cooled in the primary heat exchanger, and then all or a portion of this partially cooled stream is diverted to a warm turbo-expander, which may also be operatively coupled to and drive a compressor. The expanded gas stream or exhaust stream from the warm turbo-expander is then directed to the lower pressure column in the two-column or three column cryogenic air distillation column system. The cooling or supplemental refrigeration created by the expansion of the exhaust stream is thus imparted directly to the lower pressure column thereby alleviating some of the cooling duty of the primary heat exchanger.
The MAC compression arrangement and BAC compression arrangement require significant amount of power to achieve the required compression. Typically, the MAC compression arrangement consumes roughly 60% to 70% of the total power consumed by the air separation plant. While a portion of the air separation plant power requirement may be recovered via the above-described cold turbine arrangement and/or warm turbine arrangement which provide the supplemental refrigeration to the two-column or three column cryogenic air distillation column system, the vast majority of the power required by the air separation plant is externally supplied power to drive the multi-stage MAC compression arrangement and the multi-stage BAC compression arrangement.
Most conventional MAC compression arrangements and BAC compression arrangements as well as nitrogen recycle compressors and related product compressors are configured as an integrally geared compressor (IGC) arrangements that include one or more compression stages coupled to a single speed drive assembly, and a gearbox adapted for driving the one or more of the compression stages via a bull gear and associated pinion shafts such that all pinion shafts operate at constant speed ratios. The one or more compression stages typically use a centrifugal compressor in which the feed air entering an inlet is distributed to a vaned compressor wheel known as an impeller that rotates to accelerate the feed air and thereby impart the energy of rotation to the feed air. This increase in energy is accompanied by an increase in velocity and a pressure rise. The pressure is recovered in a static vaned or vaneless diffuser that surrounds the impeller and functions to decrease the velocity of the feed air and thereby increase the pressure of the feed air. The impellers may be arranged either on multiple shafts or on a single shaft coupled to the single speed drive. Where multiple shafts are used, a gearbox and associated lube oil system are typically required.
The conventional MAC compression arrangements further require a plurality of intercoolers provided between the multiple stages of the compressor to remove the heat of compression from the compressed air stream between each compression stage. The reason for this is as the air is compressed, its temperature rises and the elevated air temperature requires an increase in power to compress the gas. Thus, when the air is compressed in stages and cooled between stages, the compression power requirement is reduced due to closer to isothermal compression compared to compression without interstage cooling. An aftercooler, such as a direct contact aftercooler, or air chiller are also typically positioned between the MAC compression arrangement and BAC compression arrangement.
It has been suggested to replace portions of the conventional IGC arrangements with a direct drive compressor assembly arrangement. The direct coupling of the compressor and the drive assembly overcomes the inefficiencies inherent in a gear box arrangement in which thermal losses occur within the gearing between the drive assembly and the compression stages. Such a direct coupling is known as a direct drive compressor assembly where both drive assembly shaft and the impeller rotate at the same speed. Typically such direct drive compressor assemblies are capable of variable speed operation. A direct drive compressor assembly can thereby be operated to deliver a range of flow rates through the multiple compression stages and a range of pressure ratios across the compressor units by varying the drive speed.
In addition, most conventional MAC compression arrangements are designed to be optimized at a design point corresponding to a point at or near peak flow capacity. However, in many air separation plants, it has been found that compressors typically operate at their respective design conditions less than 10% of the time and, in some plants, less than 5% of the time. The peak flow capacity of the MAC compression arrangement and BAC compression arrangement will be limited by centrifugal impeller size that can be manufactured by compressor manufacturers and the allowable impeller tip speed. In conventional systems, all MAC compression stages are often driven by the same power train or drive. Therefore, once a design speed is selected for this MAC drive, there is little room to change the speed, since any speed change will impact all of the MAC compression stages as well as any of the BAC compression stages that may be also coupled to the same power train. Using this traditional design point, conventional MAC compression arrangements can often achieve a turndown (i.e. decreasing the flow rate of the air that is compressed) of only about 30% turndown using inlet guide vanes associated with one or more of the compression stages.
For any given air separation plant, while the air inlet pressure is generally constant, the ambient air inlet temperature can vary significantly from winter to summer, or even from day to night, leading to considerable variation in volumetric flow. Once the design speed is selected, there is little room to change this speed to accommodate seasonal temperature and/or production changes. Thus, the most effective compressor performance control variable, i.e., drive speed, is not a degree of freedom to use for operational control of most conventional MAC and BAC compression arrangements.
For example, to handle the required flow and the head for the summer high temperature condition, the MAC compression arrangement will need to be sized for the summer high temperature condition and inlet guide vanes will be partially closed to handle normal operating conditions. This could reduce the compressor efficiency for other operating conditions and also reduce the plant turndown range (i.e. the range from the design flow to the minimum allowable flow without compressor surge). During turndown conditions, the volumetric flow is reduced and therefore, the inlet guide vanes have to be closed further and, in some cases, compressed air may have to be vented to the atmosphere to prevent the compressors from surging. Closing of the inlet guide vanes and/or venting a portion of the compressed air both translate to waste of power and a decrease in overall plant efficiency.
Also, to optimize the air separation cycles, the compression trains of most air separation plants, including plants using direct drive compression assemblies as part of the air compression trains, are designed to provide generally constant discharge pressures to the pre-purification unit in the case of the MAC compression arrangement or pressures required by the distillation column system in the case of a BAC compression arrangement. Maintaining a generally constant discharge pressure in such air separation plants may also translate to waste of power and a decrease in overall plant efficiency across all operating conditions. There is also a need to allow for continual or periodic adjustments to the incoming feed air flow capacity and/or discharge pressures of the air compression trains without sacrificing overall air separation plant efficiency.
Accordingly, there is a continuing need to reduce the operating costs, namely power costs, associated with air compression arrangements in an air separation plant by employing effective direct drive compression assemblies as part of the air compression trains. Prior art systems that employ direct drive compression assemblies as part of the air compression trains are discussed in more detail below in the detailed description section, which includes discussion of the differences between the present invention and the prior art direct drive compression assemblies for air separation plants.